Electrical heating body and method of manufacturing the same



Nov. 19, 1929. H..| QHMANN 1,73 ,745

ELECTRICAL EA TING BODY AND METHOD OF MANUFACTURING THE SAME Filed march15'. 1928 lnventor: f/Jgo Lehman/1 i Patented Nov. 19, 1929 HUGOLOHMANN, OF BERLIN-WILMERSDORF, GERMANY* ELECTRICAL HEATING BODY ANDMETHOD OF MANUFACTURING THE SAME Application filed March 15, 1928,Serial No. 262,064, and in Germany March 19, 1927.

This invention relates to resistors for domestic and industrial uses ofall kinds. For all resistors having become known up to now a metallicresistance wire or platinum foil is used, and there exists in everycasethe invention to make the resistance as high as possible in order toreduce the consumption of current. Employing metallic resistances,especially in the shape of wire, entails very many drawbacks, amongwhich may be mentioned that they burn through easily in that they becomeoxidized'by the oxygen of the air,'and the diminution of the sectionalarea thereby caused entails overloading of the remaining sectional areawhereby the burning through is caused.

Ithas been tried to make use of carbon filaments as resistors because ofthe high specific resistance they have, and it hasalso been endeavouredto coat such filaments with metal oxides or carbonates in order toprevent them from burning, but none of the methods hitherto proposed hasbeen successful. The coating or protective layer does not prevent thecarbon filament from burning, in that,

owing to the surface energy of the carbon,

oxygen is absorbed from the air and supplied to the respective coatingor protective layer; and if this latter consists of carbonates, it isredecomposed in a short time into carbonic acid and metal oxides.

. Fig. l is a sectional view in elevation of the resistor.

Fig. 2 is an end view of, the resistor. All drawbacks hithertoencountered are i completely obviated in the present improved methodwith theresult that carbon filaments can be used as resistors for allimaginable purposes. The characteristic features of this 4 method arethese that, firstly, the filament is made, for instance, of graphite orgas coke so that its surface energy is thesmallest possible, but theground graphite or gas coke is not used directly in this state, butthere are admixed to it such substances as are suited for the formationof a layer upon the filament, viz silicium oxides, calcium oxides,aluminium oxides, or the like, which all can only with difliculty bereduced in the presence of carbon. The mixture chosen is then turnedinto a plastic composition with the aid of a suitable cementing medium,and this composition is transformed into filaments by means of asuitable press. The specific resistance of the filaments obtained can bedetermined, according to this method, either by the size of the grain ofthe metal oxide or oxides admixed to the pulverulent graphite or gascoke, or by the amount of the respective oxide or oxides, or by bothcauses, whereby it is ren- 60 dered possible to determine the specificresistance of the finished filament in very. wide limits so that thefilament itself can be regarded as an insulator.

The filament produced in this manner is dried and inserted into acircuit in whichjt is heated to a temperature surpassing the servicetemperature by about 100 0., in consequence whereof there is formed uponthe filament a layer of metal-carbonates which 7 I protects it for acertain short time from being burnt. While the current is passingthrough the filament the resistance of 'this latter changes according tothe height of the. temperature. The resistance is indicated by anohmmeter also inserted into the circuit, and the current is broken whenthe intended resistance has been reached.

But, as already said, the layer formed protects the filament from beingburnt only for a short time, as the carbonates are re-converted intocarbonic acid and metal oxide by the heating in the air.

Many experiments have disclosed the fact that the thus formed layer,even if being very 35 thin, possesses a very high insulating capacityalso at a relatively high temperature as long as it consists ofcarbonates.

When the first phase of the procedure has been carried out in theabove-described manner and the entire resistor has been provided with aninsulating la *er, a metal or an alloy is cast around it. here may beused for this purpose all metals and metal-alloys which do not melt atthe service heating, as, for instance, bronzes, aluminium, and the like.While the respective metal ;or alloy is cast around the heating bodycurrent is again conducted through it until the surface of the metaloralloy is so much cooled down thatthe latter solidifies; then thecurrent is switched ofi. Heating the body during the casting procedureis necessary in order to prevent it from being distorted owing to thedifferent coefiicients of expansion oflthe materials.

The metallic covering of the filamentprevents, by the pressure it exertsu on the filament, the decomposition of the layer of metalcarbonates,and at the same time a thermic closure is attained ,which preventsabsolutely securely oxygen from getting access .to the filament. As,therefore, porous substances permeable to gas are no more used for themanufacture of the heating body a carbon filament of the above-describedkind cannot burn, and also decomposition of the carbonate layer formedis absolutely securely prevented. And as, finally, also changes of thesectional area are perfectly impossible and the metallic enveloperenders the entire heating body very strong in mechanical respect thelife of the heating bodies produced according to this improved methodis. very long. This method permits to manufacture resistors of allshapes and for all purposes. If the heating bodies are intended forindustrial bodies metal oxides with a very high fusing point, such aszirconium oxide, thorium oxide, or uranium oxide, are admixed to thecarbon filaments.

I wish it to be understood that instead of providing the filament withonly one layer of a metal carbonate two or even more such layers may beprovided thereon. A filament with two layers of metal carbonates isillustrated by way'of example on the accompanying drawing, drawn to agreatly enlarged scale, Figure 1 showing a portion of this filament inlongitudinal section, and Figure 2 showing it in transverse section inthe plane AB of Fig. 1. On the drawing (:3 denotes thefilament properwhich is manufactured in the manner described in the preceding part ofthis specification'and consists of such substances as there stated; 0denotes an inrespective bodies electrically until a layer of a metalcarbonate has formed uponthem, and casting a layer of a metal aroundthem, substantially as set forth.

3. The method of producing resistors, consisting in mixing carbon withmetal oxides, turning the mixture into a plastic mass, forming bodies ofthis mass with the simultaneous application of pressure, inserting therespective bodies into an electric circuit, traversed by a suitablecurrent, and heating them electrically until a layer of a metalcarbonate has formed upon them, and casting a layer of a metal aroundthem, substantially as set forth. 7

4. A resistor, comprising a core of a mixture of carbon and metaloxides, a layer of a metal carbonate surrounding said core, and a layerof a metal surrounding the first-mentioned layer, substantially and forthe purpose set forth.

5. A resistor, comprising a core of a mixture of carbon and metaloxides, an insulating layer of a metal carbonate, and a protective layerof a metal, substantially as described.

6. A resistor, comprising, in combination, a core of a mixture of carbonand metal oxides, consecutive layers of metal carbonates surrounding andenclosing tightly said core, and a metallic layer cast upon and aroundthe outermost of the first-mentioned layers, substantially and for thepurpose set forth.

7. A resistor, comprising, in combination, a core of a mixture of carbonand metal oxides, a layer of aluminium carbonate, a

' layer of calcium carbonate, and a layer of a metal substantially andfor the purpose set forth. 1

In testimony whereof Iv have aifixed my signature.

' V HUGO LOHMANN.

sulating layer of aluminium carbonate, 6 an insulating layer of calciumcarbonate, and a" is the exterior, comparatively thick pro- \tectivelayer of a metal or alloy.

I claim: v V Y I 1. The method of producing resistors, consisting inmixing carbon with metal oxides,

turning the mixture into a plastic mass, forming bodies of this masswith the simultaneous application of pressure, burning the re-. spectivebodies until a layer of a metal carbonate has formed upon them, andcasting a layer of a metal around them, substantially as set forth.

2. The methodof producing resistors, consisting in mixing'carbon withmetal oxides, turning the mixture into a plastic mass,

forming bodies of this mass with the simultaneous application ofpressure, heating the

